1. Field of the Invention
This invention relates to gaseous discharge lamps which ignite at voltages that are much higher than their operating voltages and, in particular, to the igniting of such lamps.
2. Description of Related Art
Common characteristics of a gaseous discharge lamp are its negative resistance and high igniting voltage. Circuitry for powering such a lamp typically includes a current limiting means, such as a ballast, to compensate for the negative resistance, and often includes igniter circuitry for generating high-voltage pulses to ignite the lamps. Such igniter circuitry commonly includes a voltage-sensitive switch (e.g. a sidac) for effecting the continual production of the high-voltage pulses until the lamp ignites. Upon ignition, the voltage across the lamp decreases from a higher open-circuit voltage (OCV) to a lower voltage, which causes the switch to change to a non-conducting state and to effect termination of pulse production. One example of such a ballast is described in U.S. Pat. No. 5,825,139.
Igniter circuitry must be capable of starting gaseous discharge lamps despite the loading effect of parasitic capacitances associated with the lamp. Such parasitic capacitances are typically found in the wiring and fixtures via which the circuitry is electrically connected to the lamp and even in the lamp itself. Designing igniter circuitry which effectively compensates for such parasitic capacitances is difficult, because it varies significantly with, for example, the length of wiring that is used to electrically connect the igniter/ballast circuitry to the lamp. Without any compensation, the peak voltage delivered to the lamp would tend to decrease with increases in parasitic capacitance.
It is an object of the invention to provide circuitry for igniting a gaseous discharge lamp which automatically compensates for the affect of parasitic loading capacitances associated with the lamp.
It is another object of the invention to provide such compensation without substantially increasing the cost or complexity of the circuitry.
In accordance with the invention, circuitry is provided for powering a gaseous discharge lamp having a range of possible values of parasitic loading capacitance associated with it, which range extends from a lower capacitance value to a higher capacitance value. The circuitry includes a source of ignition pulses including an energy source capable of effecting charging of the parasitic loading capacitance of the higher value to at least a minimum ignition voltage of the lamp. A voltage clamping device is provided for limiting the peak voltage of the ignition pulses delivered to the lamp at the lower parasitic capacitance value to a maximum permissible voltage.
Collectively, the energy source and the voltage clamping device are capable of maintaining the peak ignition pulse voltage at a substantially constant value over a predetermined range of parasitic loading capacitance values. As another advantage, at all but the highest values of parasitic loading capacitance within the range, the ignition pulses tend to be of longer duration (and thus have increased energy levels), in comparison with known circuitry.